This invention relates generally to the cryogenic rectification of feed air and, more particularly, to the cryogenic rectification of feed air to produce low purity oxygen.
The demand for low purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Low purity oxygen is generally produced in large quantities by the cryogenic rectification of feed air. However, conventional cryogenic rectification systems for producing low purity oxygen are relatively inefficient. Moreover, such conventional systems are not effective when gaseous nitrogen or one or more liquid products are desired.
Accordingly it is an object of this invention to provide a cryogenic rectification system which can more efficiently produce low purity oxygen.
It is another object of this invention to provide a cryogenic rectification system which can efficiently produce low purity oxygen and can also effectively produce gaseous nitrogen product and/or one or more liquid products.
The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing low purity oxygen comprising:
(A) passing feed air into a higher pressure column and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched vapor and oxygen-enriched liquid;
(B) passing oxygen-enriched liquid into a lower pressure column, condensing a first portion of the nitrogen-enriched vapor, and passing at least some of the resulting condensed first portion nitrogen-enriched liquid into the higher pressure column;
(C) turboexpanding a second portion of the nitrogen-enriched vapor, condensing the turboexpanded second portion of the nitrogen-enriched vapor, and passing the condensed second portion nitrogen-enriched liquid into the lower pressure column;
(D) producing by cryogenic rectification within the lower pressure column nitrogen-richer vapor and oxygen-richer liquid, and passing oxygen-richer liquid from the lower pressure column into an auxiliary column; and
(E) producing by cryogenic rectification low purity oxygen within the auxiliary column, and recovering low purity oxygen product from the lower portion of the auxiliary column.
Another Aspect of the Invention Is:
Apparatus for producing low purity oxygen comprising:
(A) a higher pressure column, a lower pressure column having a bottom reboiler and an intermediate reboiler, and means for passing feed air into the higher pressure column;
(B) means for passing fluid from the lower portion of the higher pressure column into the lower pressure column, means for passing fluid from the upper portion of the higher pressure column to the lower pressure column bottom reboiler, and means for passing fluid from the lower pressure column bottom reboiler to the higher pressure column;
(C) a turboexpander, means for passing fluid from the upper portion of the higher pressure column to the turboexpander, means for passing fluid from the turboexpander to the lower pressure column intermediate reboiler, and means for passing fluid from the lower pressure column intermediate reboiler into the lower pressure column;
(D) an auxiliary column and means for passing fluid for the lower portion of the lower pressure column to the upper portion of the auxiliary column; and
(E) means for recovering low purity oxygen product from the lower portion of the auxiliary column.
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein, the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGrawHill Book Company, New York, Section 13, The Continuous Distillation Process. 
The term xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin. (K).
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein, the term xe2x80x9ctopxe2x80x9d when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.
As used herein, the term xe2x80x9cbottomxe2x80x9d when referring to a-column means that section of the column below the column mass transfer internals, i.e. trays or packing.
As used herein, the term xe2x80x9creboilerxe2x80x9d means a heat exchange device that generates column upflow vapor from column liquid. A reboiler may be located within or outside of the column. A bottom reboiler generates column upflow vapor from liquid from the bottom of a column. An intermediate reboiler generates column upflow vapor from liquid from above the bottom of a column.
As used herein, the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the midpoint of the column.
As used herein, the term xe2x80x9ctrayxe2x80x9d means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.
As used herein, the term xe2x80x9cequilibrium stagexe2x80x9d means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).
As used herein, the term xe2x80x9clow purity oxygenxe2x80x9d means a fluid having an oxygen concentration within the range of from 70 to 98 mole percent.